Well, huge epic battles weren't part of what Lewis was getting at in his fiction. Even in his most "combative" work, That Hideous Strength, where the conflict had the potential to get quite dramatic, the victory is won by very subtle and unexpected ways.
In Last Battle, I think Lewis' imagination was painting a very accurate picture of the condition of the Church. She is beset from without by ancient enemies and betrayed from within by self-serving members, her true leaders betrayed and in exile with only a handful of faithful followers while false leaders bully and oppress the faithful, her warriors picked off in isolation (like Roonwit) while others fall prey to despair (like the dwarfs). Just when all seems lost ("Narnia is no more."), and the remnant of the faithful determine to hold true to the vision even if there seems to be no hope - that's just when the Mighty One shows up, and we find out that He was in control all along. The direst of fates (the Stable Door, representative of death) turns out to be the gateway to paradise. The most horrible enemy (Tash) can be dismissed with a mere word. All that seemed lost and destroyed we find preserved, and all that did the destroying is banished forever.
Try this: go to Silver Chair and read Puddleglum's statement to the Witch right after he grinds his foot into the fire and breaks the spell. Then re-read the account of Tirian's decision to fight the Last Battle. See how the themes correspond? Puddleglum says, in effect, "I'm going to be true to Narnia even if there's no Narnia to be true to." So does Tirian. They have faith in the teeth of everything their senses are telling them. Puddleglum honestly expects to die in the Underworld, never to see the sun or sea again. Tirian expects to die in the Western Waste and Narnia to become an enslaved Calormene province. In both cases, their faith is rewarded.
